What they truly do is prevent his internal organs from pulling the back down. Let me explain.
Commonly, it is stated that the horse’s rectus abdominus muscle (his abs) contract in order for the horse to lift his back. This assumption makes sense - when the horse is standing still, and we rub his belly, he engages this muscle strongly and the lumbar spine elevates. However, when applied in motion, there are several issues with this assumption.
First, engaging exclusively the rectus abdominus, and releasing the back muscles, would cause the horse’s THORACIC spine (the part sitting under the rider) to drop. But we know when the horse has his back “lifted”, the thoracic spine comes UP in relation to the ground, so this cannot be the work of the rectus abdominus. In addition, we know that horses, when in good fitness, build back muscles in addition to abdominals, and have far more back muscle than abdominal muscle.
The science tells us that when a horse is moving, his rectus abdominus actually only contracts during the portion of the stride in which the horse has to counteract the movement of his viscera. Left without this muscle, the internal organs would yank downwards on the spine during the stance phase, pictured in this image. The rectus abdominus contracts the most at the center of this phase, then releases the most during the center of the suspension of the stride.
So what does all this mean? Essentially, the rectus abdominus muscle plays an absolutely vital role in helping the horse to stabilize his spine, but it does NOT lift the spine on its own while in motion. Otherwise the spine would be constantly rising and falling with its contraction alone. Did you know this, or fall for the oversimplification often spread? Comment below!